PALYNOLOGY AS A TOOL FOR CONSTRAININING THE TIMING OF THE ALASKA RANGE UPLIFT

During the Middle Miocene, (ca. 15 Ma) a warm-temperate flora (mean annual temperature [MAT] ~11° C) requiring summer-wet conditions (annual precipitation ~1000 mm) was nearly identical across Alaska, from south to north, i.e., latitudinal temperature and precipitation gradients were basically flat.

Previous studies have established that all of Alaska and northeastern Siberia endured a gradually cooling climate from Early Miocene through Pliocene. Detailed pollen data, however, from the Homerian type section near Homer in south-coastal Alaska, show that the warm and moist climate, with a MAT of ~9-11° C as indicated by warmth-loving hardwoods “thermophiles” (such as walnut, hickory, holly, hornbeam, elm, oak, and others), continued into the Late Miocene. Inexplicably, podocarps, a southern hemisphere gymnosperm family, are also present.

How could a sharp latitudinal temperature gradient have developed within Alaska in the Late Miocene? One reason might be that warm, moist Pacific air was blocked from reaching the interior by the uplift of the Alaska Range. Investigations by Russian workers of the Siberian Miocene pollen floras indicate that the Magadan coastal area shows the same kind of gradient; warm and moist near the coast, but inland, behind mountainous terrain, the pollen data show much cooler and dryer conditions.

In order fully to understand the Homerian flora, the total time span it represents, and its significance as a time indicator for the uplift of the Alaska Range, a detailed comparison with Homerian floras at more northerly locations will be important. By combining new and existing data, a north-south transect of plant records will quantify the Neogene climate differences in Alaska as a function of time, and thus, the uplift of the Alaska Range. Data from Siberia should also be studied to test this hypothesis.